Re: How does a speaker diaphragm produce the sound of 2 instruments at the same time

You can decompose any acoustic waveform into a sum of pure tones. This is called Fourier analysis, and it rests on the principle of superposition: you can simply sum up independent signals to get the resulting output, so long as the system is linear.

That is exactly the case you are asking about. The speaker output is the superposition of two frequencies, and your ear performs the Fourier analysis to separate the tones. This breaks down if the system turns non-linear (i.e., if you turn the volume up too high the sounds get distorted and fuzzy. You hear distortion products that aren't present in the original signal.) All is good when the system is linear.